A phase-locked receiver may receive two types of signals--those that are phase coherent and those that are phase incoherent. It is necessary to maintain phase coherency between the carrier of the encoded signal and the receiver's internal reference signal. A phase-locked loop is commonly employed to maintain that coherency. To maintain phase coherency, one phase-lock detector generates a control signal that is proportional to the phase difference between the received signal and the radio's internal reference signal. This phase error signal is used to maintain internal coherency with the received signal.
Another phase lock detector can serve as an indicator of the degree of coherency, and therefore the intelligibility, of the received signal. This detector has been commonly used as a Coherent Automatic Gain Control (AGC) Signal. It is called Coherent Gain Control because it is only generated when the received signal is phase coherent with the radio's internal reference signal. But, this type of gain control has several problems associated with it. Because an AGC voltage is not generated for incoherent signals, various receiver stages may become overloaded by uncontrolled incoherent signals. Further, the lack of incoherent signal gain control allows noise bursts of uncontrolled amplitude to propagate through the receiver stages during intermittent fading of the coherent signals.
Conventionally, incoherent signals are controlled by a separate AGC circuit that detects the signal envelope output by the IF amplifier. Gain control is exercised by the stronger of the response of the coherent circuit or the response of the incoherent circuit. However, the presence of IF envelope modulation may preclude the proper use of this dual AGC system.
To alleviate these problems, the present invention provides a single AGC circuit responsive to coherent signals as well as incoherent signals.